This application claims the priority of Japanese Patent Application No. 11-266242 filed on Sep. 20, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens attached to a tip portion of an endoscope; and, in particular, to an endoscope objective lens which can be used in normal viewing and close-up magnified viewing states.
2. Description of the Prior Art
For utilizing endoscopes mainly in the medical field, image transmission by use of solid-state imaging devices such as CCD and image guide fibers, and analysis and viewing of thus captured images have come into practice, whereby demands for viewing with a higher accuracy and more accurate analysis have been increasing in recent years. Therefore, as to objective lenses, it has been desired to develop an endoscope objective lens which can obtain favorable images over a wide viewing range.
When viewing an object such as a diseased part, it is preferable that not only overview but also detailed viewing of a part under magnification can be carried out. In response to such a demand, a lens with a great depth of field has conventionally been used in general so as to widen the viewing range between its far and close objects.
On the other hand, as an endoscope objective lens which is switchable between a normal viewing (wide) state suitable for overview and a close-up magnified viewing (telephoto) state suitable for partial viewing, one disclosed in Japanese Patent No. 2876252, for example, has been known. This endoscope objective lens comprises four groups consisting of negative, positive, negative, and positive lens groups successively from the object side, in which the third group is moved along the optical axis, whereby it can adjust its focusing onto an object to view at a position outside its depth of field. Depending on the position of this moving lens group, viewing can be carried out not only at both ends of the normal viewing (wide) state and close-up magnified viewing (telephoto) state, but also in the intermediate region therebetween.
Though this endoscope objective lens enables viewing from the normal viewing state to the close-up magnified viewing state, there is a limitation on the magnification in use of the objective lens in the intermediate region. Namely, though the objective lens is normally capable of taking any object distance ranging from the normal viewing state to magnified viewing state with respect to a given object to view, the magnification in use of the objective lens would be uniquely determined upon focusing onto this object distance by use of internal lens movement.
For carrying out viewing with a higher accuracy or more accurate analysis, there is a demand for an objective lens having a higher degree of freedom in its magnification in use even in the viewing state in the intermediate region between both ends.
In view of such circumstances, it is an object of the present invention to provide a compact endoscope objective lens which can arbitrarily change its magnification in use in an intermediate region between both ends of its normal viewing state and close-up magnified viewing state, while various kinds of aberration are favorably corrected therein.
The endoscope objective lens in accordance with the present invention comprises, successively from an object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a negative refracting power, and a fourth lens group having a positive refracting power; wherein two lens groups consisting of the third lens group and one of the second and fourth lens groups are moved along an optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (1) and (2):
2.0 less than xcex21T/xcex21W less than 10.0xe2x80x83xe2x80x83(1)
1.2 less than fT/fWxe2x80x83xe2x80x83(2)
where
xcex21T is the magnification of the first lens group at the telephoto end;
f1W is the magnification of the first lens group at the wide end;
fT is the focal length of the whole lens at the telephoto end; and
fW is the focal length of the whole lens at the wide end.
Preferably, in the endoscope objective lens in accordance with the present invention, two lens groups consisting of the second and third lens groups are moved along the optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (3), (6), and (7):
0.1 less than |d2|/f2 less than 1.0xe2x80x83xe2x80x83(3)
0.7 less than f2/fW less than 4.0xe2x80x83xe2x80x83(6)
xe2x88x923.0 less than f3/fW less than xe2x88x920.5xe2x80x83xe2x80x83(7)
where
d2 is the moving distance of the second lens group from the wide end to the telephoto end;
f2 is the focal length of the second lens group;
f3 is the focal length of the third lens group; and
fW is the focal length of the whole lens at the wide end.
Preferably, in the endoscope objective lens in accordance with the present invention, two lens groups consisting of the third and fourth lens groups are moved along the optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (4), (7), and (8):
0.1 less than |d4|/f4 less than 0.8xe2x80x83xe2x80x83(4)
xe2x80x83xe2x88x923.0 less than f3/fW less than xe2x88x920.5xe2x80x83xe2x80x83(7)
1.0 less than f4/fW less than 2.5xe2x80x83xe2x80x83(8)
where
d4 is the moving distance of the fourth lens group from the wide end to the telephoto end;
f3 is the focal length of the third lens group;
f4 is the focal length of the fourth lens group; and
fW is the focal length of the whole lens at the wide end.
Preferably, in the endoscope objective lens in accordance with the present invention, a stop is disposed between the second and third lens groups; the endoscope objective lens further satisfying the following conditional expression (5):
0.4 less than xcex23.4T/xcex23.4W less than 2.5xe2x80x83xe2x80x83(5)
where
xcex23.4T is the composite magnification of the third and fourth lens groups at the telephoto end; and
xcex23.4W is the composite magnification of the third and fourth lens groups at the wide end.